Conductor for heat exchange apparatus



Sept; 1 1931,

' E. B. FREEMAN CONDUCTOR FOR HEAT EXCHANGE APPARATUS Filed Dec.

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Patented Sept. 1-, 1931 1 UNITED STATES- PATENT orrlcr.

ERNEST B. FREEMAN, OF NEWTON, MASSACHUSETTS, ASSIGNOR TO B. E.STURTEVANT COMPANY, OF HYDE PARK, MASSACHUSETTS, A. CORPORATION OFMASSACHUSETTS CONDUCTOR FOR HEAT EXCHANGE APPARATUS Application filedDecember 24, 1926. Serial lllo. 156,902.

The present invention relates to conductors for heat exchange apparatus,and more particularly, to conductors which are required to be subjectedto the corrosive action of .water 5- and heated gases.

In heat exchange apparatus such as economizers and super-heaters, thetubes are preferably of steel, in order to afford -sufficient mechanicalstrength to resist the usual stresses to which they are subjected.However, steel tubes are open to the serious objection that they arequickly corroded, not only by the action of the heated gases, but alsoin the case of economizers, by the feed Water at certain temperatures.In order to resist corrosion, it has been proposed to cover the tubeswith cast iron ackets which provide a measure of protection against theaction of gases on the exterior. Aside from the disadvantage of undulyincreasing the weight of the structure, the ackets afi'ord no adequatesolution of the problem because they cannot protect against internalcorrosion by the water, and the best remedy for this latter defect whichhas been devised for economizers consists in regulating the temperaturesthroughout the apparatus so that internal corrosion is localized in aportion of the tubes. Moreover, in the construction of such apparatus,another serious problem is encountered, namely, the erosion or pittingof the surface by mechanical impact therewith of swiftly moving solidparticles which are inevitably present" in the gas stream. In thisrespect, a tube with cast iron jackets offers no advantages over aplainsteel' tube, except such as may be provided by the greaterthickness of its walls.

The object of the present invention is to provide a strong andrelatively light conductor for a heat exchange apparatus which willsatisfactorily resist both external and internal corrosion under allconditions and which afi'ords adequate protection against erosion byimpact of solid particles.

With this object in view, the principal feature of the present invention,consists in a heat exchange conductor preferably of wrought steel,which is provided both internally and externally with a thin,-tough,continuous and adherent coating of corrosion resisting soft metal. Thiscoating is conven-' iently of lead which has high resistance tocorrosion and which may be readily applied to the tube, but may be ofany alloy of soft metals which has the desired effect of preventingattack on the tube by the water or gases. This coating completely coversall portions of the tube, even though the tube may be formed with anextended surface to increase the heat transmission. The coating isextremely thin, being of a thickness of approximately one one-thousandthof an inch or less, so that its presence does not add materially to theweight of the structure or increase the stresses which the tube mustwithstand.

The accompanying drawing is an elevation partly in section of thepreferred form of tube for heat exchange apparatus acccording to thepresent invention.

The illustrated embodiment of the invention consists of an economizerconductor adapted for the heating of feed water for boilers. The wateris passed internally through the tube and the exterior of the tube isswept by heating gases passing from the boiler to the stack. Referringto the drawing, the conductor consists of a tube 2 of wrought steel ofsuflicient thickness to withstand the internal pressure of the water.The conductor is formed with an extended surface consisting of a seriesof regularly spaced flanges or ,discs 4 which are mounted on the tube.In order to secure the discs firmly to the tube and to insure a highquality of heat transmission between them, the tube is internallyexpanded by passing therethrough a tool of slightly larger diameter thanthe interior of the tube. This action serves to expand the metal of thetube wall so that the flanges are slightly embedded in the wall, thussecuring a firm contact. The action of passing the expanding toolthrough the tube results in forming slight bulges on the tube wallbetween adjacent flanges which action also enhances the security ofattachment of the tube and discs. The bulges are exaggerated in thedrawing for purposes of illustration, but in practice, are of onlysuffilllll cient magnitude to be barely perceptible upon looking throughthe tube.

The conductor, after attachment of the discs, is next subjected to theoperation of coating it with soft metal. Preparatory to the coatingoperation, the conductor is pickled in a suitable bath which isconveniently sulphuric acid. From the pickling bath the conductor ispassed to an alkaline bath, preferably a solution of ammonium chloride,which neutralizes the .acid. It is then immersed in the molten softmetal. This bath is preferably commercially pure lead which has beenfound to be most suitable for the purpose of resisting the corrosive anderosive actions of the gas and water in actual use. In order to causefirm adherence of the lead to the conductor, the surface of the bath isprovided with a suitable fiux, such as zinc chloride. The conductorfloats on the bath and may be occasionally forced below the surface androtated manually in order to afford contact of the lead with all partsof the conductor. During this operation, it is advisable to dustammonium chloride occasionally over such portions of external surface asappear to resist the .deposit of the lead film thereon. The action ofthe flux on the surface of the bath has been found to be suflicient forcausing adequate coating the interior of the tube. After the tube hasbeen completely coated, as will be evidenced by the appearance of theshiny continuous coating of lead on the exterior the conductor isremoved from the bath and is quickly immersed in a quenching bathconsisting of kerosene or other oil. The quenching bath improves theappearance of the tube, imparting the eifect of a smooth, continuous,integral structure, and at the same time enhances the bond between thelead and the steel. After cooling, the tube is ready for use and may beemployed for the construction of any heat exchange apparatus such as aneconomizer, super-heater or condenser.

The lead coating is illustrated in the drawing by the heavy black line6. The thickness of the coating is necessarily exaggerated for purposesof illustration and as previously stated, is not of an average thicknessgreater than one one-thousandth of an inch. There is, however, asomewhatgreater deposit 01 lead at the junction of the tube and discs,appearing in the form of fillets indicated at 8. This increasedthickness at the point of attachment of the discs to the tube is ofimportance, because it provides additional protection at the pointswhere corrosion is most likely to occur, and where, by virtue of itstendency to impair the contact of the discs with the tube, it wouldprove to be most serious.

The coating of lead or other corrosionresisting soft metal, has beenfound not only to provide all necessary protection against externalcorrosion by heating gases or internal corrosion by the feed water, butalso to afford protection against destruction by impact of solidparticles therewith. These solid particles which are present in all fluegases, have a detrimental effect on the usual heat exchange tubes, butpeculiarly the soft metal film is not affected by them to anyperceptible degree. In addition to its long life and high durability,the conductor of the present invention possesses a high efficiency ofheat transfer because of the intimate connection of the flanges with thetube.

What .I claimis 1. A conductor for heat exchange apparatus comprising ametal tube, discs secured to the tubefor providing an extended surface,and a tough, continuous and adherent coating of lead on the interior andexterior of the conductor and forming a deposit of greater thickness atthe junction of the tube and discs than elsewhere.

2. A conductor for heat exchange apparatus comprising a steel tube,extended surface means attached to the tube and spaced thereon to exposeportions of the tube proper, and a thin, continuous, adherent coating oflead applied while in a molten condition to the exterior of the tube andthe extended surface means and sealing the joints between the tube andthe extended surface means.

3. A conductor for heat exchange apparatus comprising a steel tube,disks attached to the exterior of the tube at spaced intervals,

and a thin, continuous, adherent coating of lead covering the interiorand exterior of the tube and the disks and sealing the joints betweenthe tube and the disks.

4. A conductor for heat exchange apparatus comprising a metal tube,extended surface means attached to the exterior of the tube and spacedthereon to expose portions of the tube proper, and a thin continuousadherent coating of soft metal, resistant to erosion by impact of solidparticles on the interior and exterior of the conductor and forming adeposit of greater thickness at the junction of the tube and theextended surface means than elsewhere. 1

In testimony whereof I have signed my name to this specification.

ERNEST B. FREEMAN.

